1. Field of the Invention
This invention relates to a vertical synchronizing signal detector used for the video tape recorder for recording or reproducing a color television signal having a VIR signal.
2. Description of the Prior Art
In the video tape recorder for recording and reproduction without guard bands, crosstalks occur between color signals. Therefore, a comb-shaped filter having a delay circuit of one horizontal period is provided in the color signal system for the purpose of special signal processing, thus removing the crosstalks. In the case where the color television signal having a VIR signal inserted in the 19th horizontal period is recorded or reproduced in a video tape recorder having a comb-shaped filter, the video tape recorder fails to produce a correct VIR signal. In reproducing the output signal of the video tape recorder by a television receiver which automatically corrects the color level or hue by means of the VIR signal, the color signal is controlled by an incorrect VIR signal, thus making it impossible to produce an image of a correct color tone. For this reason, Japanese Patent Laid-Open No. 149430/77 discloses, for instance, a system in which a VIR signal is adapted to be produced also in the 18th horizontal period. This VIR signal is inserted in the 18th horizontal period thereby to perform the right recording and reproduction of the VIR signal. Further, a system in which the VIR signal inserted in the 19th horizontal period is erased is disclosed in Japanese Patent Laid-Open No. 150932/77. In both cases, however, unless the horizontal periods are counted correctly, the VIR signal may be inserted in an undesired horizontal period or the VIR signal in the 19th horizontal period may fail to be erased, thus making it impossible to produce a reproduced image of correct color tone. In order to count the horizontal periods correctly, it is necessary to detect the field accurately. For this purpose, a vertical synchronizing signal must be detected accurately.
In an ordinary television receiver, the separator circuit for separating a vertical synchronizing signal from a composite synchronizing signal including the horizontal synchronizing signal, equalizing pulses and vertical synchronizing signal comprises a filter and a discriminator circuit for discriminating the filter output level. This separator circuit is such that the threshold level and the time constant of the filter are not stable against changes in temperature, so that the vertical synchronizing signal cannot be detected accurately.
With the intention to obviate this problem, a vertical synchronizing signal detector circuit for detecting a vertical synchronizing signal by use of a digital circuit is disclosed in Japanese Patent Laid-Open No. 82121/78. In this known vertical synchronizing signal detector, by utilizing the difference in pulse width between the horizontal synchronizing pulse, equalizing pulses and vertical synchronizing pulses, clock pulses of predetermined frequency generated during respective pulse widths are counted, so that from the resulting counts, the vertical synchronizing pulses are detected. In the case where this known vertical synchronizing signal detector circuit is used in an ordinary video tape recorder not provided with a circuit for eliminating noise pulses mixed with the composite synchronizing signal, the noise pulses are also counted with the result that a horizontal period in which noise pulses are generated may be wrongly taken as vertical synchronizing pulses. In view of the interlaced scanning in the television receiver, this known vertical synchronizing signal detector circuit detects the starting or ending point of the vertical synchronizing pulse. Therefore, between successive fields, the generation of the detection signal is displaced by 1/2 horizontal period, thus making it impossible to correctly count the horizontal periods.